DEVELOPMENT OF A GAS-DIFFUSION PROBE FOR THE DETERMINATION OF METHANECONCENTRATIONS AND DIFFUSION CHARACTERISTICS IN FLOODED PADDY SOIL

A probe for the measurement of dissolved CH4 in anoxic methanogenic en vironments was developed. The probe was based on the diffusion of diss olved CH4 through a silicone membrane into a gas space at the end of t he probe. This gas space was flushed with N-2 and analyzed gas-chromat ographically for CH4. The probe had a spatial resolution of <1.3 mm, t he detection limit was about 20 mu M CH4, the precision of the measure ment was 9%, and consecutive measurements could be made every 4 min. M emory effects after analysis of high CH4 concentrations could be avoid ed by flushing the probe with N-2 between each measurement. The probe was sensitive for water movement and, therefore, was calibrated in an artificial sediment of glass beads (100 mu m diam.) immersed by aqueou s solutions of known CH4 concentrations. Sensitivity of the probe for changes in the sediment's porosity could not presently be excluded. Th e probe was used to measure vertical profiles of dissolved CH4 in micr ocosms of anoxic Daddy soil. The vertical CH4 profiles measured with t he probe compared fairly well with those measured after an extraction procedure. The profiles clearly showed that CH4 was produced in deeper layers and diffused upwards to be consumed in the oxic top 2 mm soil layers. The probe was also used to determine the diffusion coefficient of CH4 in an inactivated paddy soil microcosm using a set-up which al lowed modelling of a measured CH4 concentration profile using Fick's 2 nd law.